1. Field of the Invention
The invention relates to a device for adjusting the height of the backrest of an office chair, and more particularly to a backrest and a waistrest both of which are height-adjustable and achieve an automatic drop in reaching a predetermined height.
2. Description of the Related Art
As shown in FIG. 1, a conventional office chair 10 includes a seat body 12 which is supported by an upright rod on a base 11 with a plurality of rollers. A seat back 13 is positioned upright at the tail of the seat body 12. An arched and streamlined headrest 15 is mounted at the top of the seat back 13 via a pair of support bars 14. The height of the headrest 15 is adjustable by the fixing position of the support bars 14 and the seat back 13. The optimal resting angle for the back and the neck depends on different body shapes, personal requirements and needs of the users when they sit on the chair. The conventional fixed type headrest design obviously fails to meet the above-mentioned requirements so that it is impractical.
Besides, the conventional office chair is often provided with a waistrest 16 on the seat back. The waistrest 16 is indirectly driven by a transmission set (not shown) within the seat back 13 to impart a telescopic movement to an internal frame plate. The drive position is located at the side of the seat back 13 such that the assembly and the operation will do great harm to the whole appearance of the chair. When not used and removed, it occupies a considerable mounting space. Moreover, the waistrest which is mounted and telescopically moved within the seat back is unable to achieve the expected supporting or resting effect due to the insufficient resilience and the too small distance of the telescopic drive movement. In addition, the waistrest mounted and telescopically moved within the seat back 13 requires the additional transmission set for driving an internal frame plate in a telescopic movement, thereby causing the cost burden of the component assembly.
According to U.S. Pat. No. 7,303,232B1 filed by the applicant of the invention, a “BACKREST ADJUSTING DEVICE FOR OFFICE CHAIRS” is disclosed for eliminating the drawbacks of the prior art. As shown in FIG. 2, a device for adjusting the angle of a waistrest 17 and for adjusting the height and the depth of the headrest 18 is provided to resolve the above-mentioned drawbacks and further to achieve the expected effects.
According to the above-mentioned conventional structure, the backrest 19 is ridigly attached to the seat body 12 such that the height thereof is not adjustable. As a result, it is not possible to adjust the chair to an optimal and ergonomic state according to the personal requirements. Thus, there is still room for improvement.